The present invention relates to an electron beam lithography system for rendering fine patterns using an electron beam.
Generally, electron beam lithography systems are known for such characteristics as (1) an ability to form fine patterns, (2) possession of a pattern generating function, and (3) a capability of correcting distortion. With these features in their arsenal, the systems have been used extensively in developing LSIs and fabricating masks for optical exposure. While excelling in fine working, the electron beam lithography systems are nevertheless confronted with a physically unavoidable bottleneck known as the proximity effect experienced when VLSI patterns are rendered on the submicron order.
FIG. 16 is a schematic view illustrating the principle of the proximity effect. In FIG. 16, an electron beam 72 is incident on a substrate 70 over which an electron beam resist 71 has been applied. The proximity effect is caused primarily by two kinds of electron scattering: forward scattering and backward scattering. Electrons emitted by an electron gun and incident on the electron beam resist 71 first collide with component atoms of the resist 71 and are scattered thereby continuously while discharging their energy. The process is called forward scattering. Most of the incident electrons penetrate the substrate, get scattered thereby and return partially to the resist 71. This process is called backward scattering.
These processes result in the incident energy being accumulated not only in the irradiated (i.e., exposed) region but also elsewhere. Hence an insufficient dose (exposure) likely to occur in rendering fine patterns. Conversely, excess energy tends to accumulate in space between figures of a large pattern. Consider a case where an electron beam is used to render a pattern made of a plurality of figures. In that case, if the figures are numerous and are clustered in close proximity, they tend to be rendered thick; if the figures are sparsely distributed and isolated from one another, they are likely to be rendered thin. The phenomenon is called the proximity effect.
One technique for correcting the proximity effect is disclosed illustratively in Japanese Published Unexamined Patent Application No. Hei 03-225816. The disclosed technique involves calculating a rendering area of each of regions divided by rendering data and thereby creating a dose (exposure) map. The exposure map is referenced for each shot. The exposure is increased for patterns of low rendering densities and lowered for patterns of high rendering densities, whereby each pattern is rendered at an optimum exposure level in keeping with the rendering density in effect.
Other techniques have been proposed to correct the proximity effect. For example, U.S. Pat. No. 4,463,265 discloses a so-called ghost method whereby a defocused electron beam is used to render a monochromatically reversed pattern of a target figure for correcting the proximity effect. Japanese Published Examined Patent Application No. Sho 63-14866 describes a method for obtaining levels of exposure at opposing ends of a dimensionally corrected pattern so that the pattern ends will have intensity ratios lower than a predetermined value. Japanese Published Examined Patent Application No. Sho 62-46059 discloses a method for acquiring a ratio of an irradiation density at which a resist pattern of the same size as that of a target rendering pattern is obtained, to an irradiation density at which a predetermined residual film is provided, so that the target pattern is rendered at a suitable irradiation density and in a suitable size for the residual film to be formed.
The technique disclosed in Japanese Published Unexamined Patent Application No. Hei 03-225816 is characterized in that it initially carries out mock rendering to create an exposure map and then performs actual rendering by referring to the exposure map thus created.
FIG. 17 is a schematic block diagram illustrating the concept of the above technique. Rendering data that are input to an input unit 81 comprise exposure data as well as position and shape data about each of shots computed by a figure disassembling and reassembling correction unit located upstream of the input unit 81. The exposure map is created by first carrying out mock rendering. When rendering data (shot shapes and positions) are received from the preceding stage, an area value computing unit 82 calculates the area of each shot.
As shown in FIG. 18, the rendering data are divided into square meshes having a side length of xcex1 each. To a mesh containing a center 91 of each shot 90, the area of the shot in question is added cumulatively. The result is stored temporarily in a memory portion called a partial memory 83. The process is repeated. The area value held in the partial memory 83 is transferred to an exposure map memory 84. The data retained in the exposure map memory 84 are subjected to such processes as data smoothing by a smoothing filter 85. The processed data are placed back into the exposure map memory 84.
Actual rendering is then performed as follows: the same rendering data are again received from the preceding stage. Given the shot position and shape data, an address computing unit 86 calculates an address in the exposure map. The area value for the calculated address in the exposure map memory 84 is converted simultaneously to a level of exposure by an exposure converting unit 87. An adder 88 adds or subtracts the exposure level to or from the exposure of each shot. The output of the adder 88 is forward downstream through an output unit 89. In this manner, shots included in large-area meshes are assigned low levels of exposure while those in small-area meshes are given high levels of exposure, whereby desired correction is supposed to be accomplished.
Where the exposure map is created by adding the area of each shot to a mesh containing the center of the shot in question, as described above, problems occur if the shot center is located close to a mesh boundary. FIG. 19 is a schematic view outlining such problems experienced illustratively when three meshes contain a number of shots.
In FIG. 19, shots 1 and 3 are significantly close in size to their corresponding meshes while a shot 2 is appreciably smaller than its corresponding mesh. The shot 1 straddles the meshes 1 and 2. The center C1 of the shot 1 is included in the mesh 1 close to the boundary between the meshes 1 and 2. The shot 2 together with its center C2 is included in the mesh 2. The shot 3 straddles the meshes 2 and 3. The center C3 of the shot 3 is included in the mesh 3 close to the boundary between the meshes 2 and 3.
In the above makeup, the proximity effect works in such a manner that space between shots tends to be narrowed. Thus to correct the adverse effect requires reducing the exposure of the shot 2. With the above conventional method, however, the area value is accumulated at the center of each shot so that, as shown in the lower part of FIG. 19, all area value of the shot 1 is added to the mesh 1 and the entire area value of the shot 3 is added to the mesh 3.
Meanwhile, the mesh 2 that needs to have the largest possible area value is only supplemented by the area value of the shot 2. As a result, the meshes 1 and 3 are given large area values while the mesh 2 is assigned a small area value. When this kind of exposure map is used for exposure correction, the shot 2 in the mesh 2 is corrected to have a raised level of exposure and the shots 1 and 3 are corrected to have lowered levels of exposure. This type of exposure correction is contrary to what is expected of the corrective process implemented. Exposure correction remains unavailable.
FIGS. 20 and 21 illustrate specific simulations for purpose of illustration. FIG. 20 shows rendering data representing a 60 xcexcmxc3x9760 xcexcm square. FIG. 21 depicts a three-dimensional exposure map created by dividing the rendering data into 3.0 xcexcmxc3x973.0 xcexcm shots. In this exposure map, each mesh measures 5.12 xcexcm by 5.12 xcexcm within a range of 20 by 20 meshes. The obtained values are each given as a percentage of the shot area in each mesh, i.e., as area ratios.
Seen in FIG. 20, the exposure map may be expected to have a substantially uniform area ratio distribution in the figure-containing region. In fact, simulation results in a distinctly undulating exposure map as shown in FIG. 21. If rendering were performed by use of this map, exposure errors of up to xc2x140% would occur. Such extensive exposure errors would completely nullify any expectations of enhanced accuracy, and the attempt to correct the proximity effect would come to nothing.
At present, the accuracy of proximity error correction is typically raised by reducing the shot size. Illustratively, dividing rendering data into 0.64 xcexcmxc3x970.64 xcexcm shots allows an exposure map to be created to xc2x15% precision. When the undulating map thus created is subjected to smoothing (e.g., by use of an averaging filter in specific mesh regions), the error may be further reduced to xc2x11.5% or less.
The above technique has its share of disadvantages. Reducing the shot size increases the number of shots in the same figure rendered. Illustratively, if the shot size is reduced from 3.0 xcexcmxc3x973.0 xcexcm to 0.64 xcexcmxc3x970.64 xcexcm, the number of shots is increased by a factor of about 22[=(3.0/0.64)2]. Because the time required to create an exposure map is substantially proportional to the number of shots involved, the processing time would theoretically by 22 times as long as the original duration. Although the degree of increases in processing time may not be so dramatic with actual rendering data, this is an illustration of how much time is needed to enhance mapping precision.
It is therefore an object of the present invention to overcome the above and other deficiencies and disadvantages of the prior art and to provide an electron beam lithography system capable of creating a highly precise exposure map without reducing shot sizes.
The foregoing and other objects of this invention are achieved illustratively by an electron beam lithography system which divides shots by mesh boundaries and splits an area in a given mesh for cumulative addition of the split area value to an adjacent mesh. The center of each shot is not considered when an exposure map is created.
FIG. 1 is a conceptual view showing the principle of this invention schematically. In FIG. 1, a shot 1 straddles meshes 1 and 2, a shot 2 is inside the mesh 2, and a shot 3 straddles the mesh 2 and a mesh 3. In such a case, the area value of the shot 1 is divided into the meshes 1 and 2 and the area value of the shot 3 into the meshes 2 and 3, whereby the area value of each mesh is supplemented cumulatively. As a result, as shown in the lower part of FIG. 1, the mesh 2 comes to have a larger area value than that of the mesh 1 or 3. Using this kind of exposure map thus makes it possible to reduce the exposure of the shot 2 so as to correct the proximity effect.
In carrying out the invention and according to one aspect thereof, there is provided an electron beam lithography system comprising: exposure map creating means which, based on positional relations between meshes dividing a region to be rendered by an electron beam on the one hand and shots to be rendered by the electron beam on the other hand, creates an exposure map by calculating an area density from a shot area included in each of the meshes; and proximity effect correcting means for correcting a level of exposure for each of the shots by referencing the exposure map so that each shot is exposed at the corrected level; wherein the exposure map creating means includes judging means for judging whether or not each shot straddles a plurality of meshes.
In a preferred structure according to the invention, based on positional relations between coordinates of two diagonally positioned end points of each shot on the one hand and mesh boundaries on the other hand, the judging means may judge whether the shot in question straddles the plurality of meshes. The exposure map creating means may preferably divide each shot straddling the plurality of meshes by boundaries of the meshes so that either area values or area densities of divided shots included in each mesh are added to the mesh in question.
In another preferred structure according to the invention, the electron beam lithography system may further comprise Nxc3x97M memories for accommodating either area values or area densities of shots, N representing a maximum number of divided shots in a direction of one boundary of a given mesh, M denoting a maximum number of divided shots in a direction of another boundary of the mesh in question. This structure is preferred for its ability to boost processing speeds. If the area value of a given shot is divided into Nxc3x97M portions and if the divided values were stored into a single memory, that memory would have to be accessed Nxc3x97M times because area values vary in different meshes having obviously different addresses.
If Nxc3x97M partial memories are provided with the same address, write operations are performed in a relatively easy manner, with the divided area values stored per shot and written simultaneously to adjacent addresses of each mesh. Upon retrieval, Nxc3x97M data are required to be read simultaneously. The requirement is met by an adding function provided downstream of the partial memories. Preferably, when either an area value or an area density of each shot is divided for a plurality of meshes in order to store the divided values or densities into the memories, either the divided shot area values or the divided shot area densities included in each mesh may be set simultaneously to different addresses in different memories so that when data are to be retrieved from the memories, the data are read from the same address of all memories. The electron beam lithography system may further comprise a function for adding up a plurality of data retrieved from the same address in a plurality of the memories.
It is also possible to divide the region to be rendered by the electron beam into mesh groups each having Nxc3x97M meshes, each mesh group being assigned a single address in each of Nxc3x97M memories. Preferably, the electron beam lithography system may further comprise the Nxc3x97M memories assigned the same addresses as those of the Nxc3x97M meshes constituting each of the mesh groups dividing the region to be rendered by the electron beam. The system may also comprise, preferably, selecting means for selecting a desired memory as well as a desired address therein from among the Nxc3x97M memories in accordance with the address of a given mesh.
In a further preferred structure according to the invention, the selecting means may select the memory into which to store either the area value or the area density of the mesh in question at an address (m, n) on the basis of a remainder from a formula of n/N and a remainder from a formula of m/M, the selecting means further selecting the address based on a quotient of the formula of n/N and on a quotient of the formula of m/M. With this preferred structure, a single memory may be selected by use of a bit string of up to (Nxc3x97M)/2 bits starting from the least significant bit of a mesh address, and an address in the selected memory may be selected using higher-order bits than the (Nxc3x97M)/2 bits.
According to another aspect of the invention, the electron beam lithography system has an exposure map made of Nxc3x97M memories.
These and other objects, features and advantages of the invention will become more apparent upon a reading of the following description and appended drawings.